Rust Monster Card art by Joe Bilicic

Rust Monster Origins, Biology, & Lore

by Warren McHenry

“On the first swing my axe was sturdy. On the second, it fell apart like soggy bread. This was solid metal, mind you. Quality, enchanted steel with a razor edge. Or it was, until it touched that thing.”

—Zuhr Chindik, Ruin Explorer

Origins

Not all monstrosities begin life as twisted horrors.  Most are born innocuous, no more terrifying than the common salamander.

Take the simple Chuloch, an odd-looking but harmless creature whose shape approximates that of a garden slug.  Chulochs live silent lives deep underground, where they feed on natural iron deposits.  They grow no larger than a man’s arm, and the iron within a simple dagger could sustain one for years.

Alchemists during the Elder Age were fascinated by the Chuloch’s ability to digest metal.  A brief but intense ‘Chuloch Rush’ followed, during which time Chulochs were collected by the thousands and subjected to intense experimentation.

Years of study led to a disappointing but inescapable result:  No matter what metals or magic were fed to a Chuloch, they could not transform it into gold.

Despite interest by arms-makers in the alloys found in Chuloch skin, most research was quickly abandoned.  So the metal-eating slug found itself once again relegated to that class of notably dull creatures unworthy of examination.  Bestiaries considered them a waste of ink, when more impressive monsters remained to be cataloged.

And so the Chuloch was ignored, until Hlin Mool, a Dwarven Tinkerer-Mage, again realized its potential.

Mool wished to create a beast of burden, not for the field, but for the mine. Upon rediscovering the Chuloch, her notes gush:  “At last, my living plow.  The perfect organism for tilling stone fields and harvesting ore crop!”

For years, Mool worked with Chulochs in her cave-laboratory, increasing their size and their ability to process ore.  Success was elusive at first, but her journals soon filled with peculiar accounts:

“Specimen now the size of a dog.  Mines ten times as fast as the common type! Snapped at me today as I closed the cage.”

“New generations quickly lose taste for raw ores and seek out fabricated metals instead.  Will mine only when starving.”

“Latest varieties are larger and more aggressive.  Running low on iron plate.”

Her last entry is dated Sol-Monap, Mud Month, 1294:

“Blardus and Blun cornered me in the creche today.  Nearly lost a finger and my gold ring to Blardus’ beak.  Blun was quite interested in the blood that dripped onto the stone, and began to mine where it pooled.  Used the last of the iron scraps and my mail shirt to coax them back into the deep pens.  Need more iron for the flock, or risk losing progress made here.  Close to a breakthrough.  I can feel it.”

Years later, Mool’s desiccated body was discovered in her laboratory.  Her notes were intact, but every scrap of metal in the area–latches on cupboards, buckles on belts, even the strings on her mandolin–had been consumed.

Mool’s lab was crisscrossed with hundreds of narrow tunnels, but no trace of her Chulochs was found.

Biology

Rust Monsters range in size from that of a housecat to that of a small horse, depending on age and diet.  They have the appearance of a horse’s body riding atop six to twelve pairs of insectile legs, with a sturdy neck, and a bird-like head possessing a pair of opposed, diamond-hard beaks capable of grinding rocks and shearing metal.

Feathery gills conceal hundreds of black eyelets that encircle the head, granting superior darkvision in every direction.  A set of antennae at the rear of the head work to detect metal.  It is believed these antennae provide Rust Monsters with excellent direction sense, and with the ability to detect magic at short distances.

Four sinewy tentacles with club-like tips protrude from the chest and neck.  These extend outward up to three times the length of the body, and can bludgeon targets with great strength.  The tentacles bristle with hooked barbs that anchor the monster to steep walls and make it an excellent climber.

A Rust Monster’s skin is fleshy and thick, lacking scales, but infused with metal fibers that provide it with natural armor.  In addition, Rust Monsters secrete a permanent layer of corrosive slime that dissolves metal and stone, and inflicts damage to living flesh.  They fling and spit this slime at predators, and quickly regenerate whatever they use.

Behavior

Rust Monsters are metallovoric, consuming metals and ores.  As a result, they have no natural prey and are instead drawn to areas rich in metal content, such as mines, or pinpoint sources, like armor and ingots.

Using their corrosive slime, Rust Monsters dissolve the rock surrounding metal deposits.  They use their powerful beaks to break apart the slag, revealing the ore inside.  Fabricated metal is chewed and directly ingested, bypassing the slime process.

Rust Monsters will defend themselves when attacked or provoked, but their response is typically wild.   In combat, they often appear panicked, lashing out with frenzied force until their attackers retreat or stop moving altogether.

When Rust Monsters detect a high-quality metal source or a particularly interesting magic item, they will pursue it with the intensity of a starving animal.  They will attack the bearer, and continue to track the item until either it is consumed, they are driven off or killed, or they find a new target.  It is sometimes possible to evade pursuit by dropping the item of interest and fleeing the area. It is also possible to distract a Rust Monster with a better meal, such as a large amount of dropped coin or a stronger magic item.

Despite their name, these creatures do not feed purely on ferrous objects.  They find most metals palatable to some degree, especially precious metals like silver and gold.  However, the taste of rare metals like Mithral and Adamant seems to have a deleterious effect, and can cause the monster to sputter and flee.  Interestingly, Rust Monsters are confused by the crystalline structure of glass.  Their antennae identify it as food, but they cannot consume it, instead becoming aggressive and distressed by encounters with the material.

Rust Monsters are solitary, and territorial with members of their own species.  They will cohabitate only in spacious areas where metal is plentiful.  Mines, both active and abandoned, make ideal lairs for armories of Rust Monsters.

Since they do not compete for food with most creatures, and are themselves inedible due to their high metal content, Rust Monsters have few predators.

Life Cycle

Both Rust Monsters and their originating Chulochs reproduce asexually.  Their reproduction cycles are dependent on maturity, subtype, and food supply.  The most common form will produce two or three viable offspring in a year, while a the rarest might reproduce once in a decade.

When metal is not available, Rust Monsters extract limited nutrition from stone and earth.  This alternate food supply is inadequate for growth, but sufficient to sustain them during times of scarcity.  The longer a Rust Monster goes without first-rate food, the more aggressive its response to metal will be.

When a Rust Monster dies, others in the area detect and scavenge its body for the metal content.  The accumulated metal within a mature Rust Monster can be substantial, sometimes up to a quarter of its body weight.  Larger and rarer variants sometimes contain exotic metals, or even whole items yet to be digested.  For this reason, Mountain Dwarves, Duergar, Drow, and subterranean Goblinoids have long hunted Rust Monsters in hopes of harvesting a treasure.

Mythology & Lore

Living almost entirely underground, and in close proximity to sources of ore, Chulochs have been interacting with Dwarves for millennia.  Dwarven histories are filled with references to Chulochs.  Sometimes they are depicted as harbingers of devastation, and at others they are revered as caretakers of the deep.  Dwarven legend hints at titanic variants of the creature, large enough to hollow out underground caverns.

Substitute “Rust Monster” for “Chuloch” in the Dwarven legendarium, and one understands how their lore was uniquely primed for the monster’s arrival.

One song from Old Stories is especially notable, in which ancient Dwarves laid down their steel and took up wood and stone against a new threat from the Undermere.

Verses 31 and 32, translated from Old Dwarvish by the Gnome seer, Fidalian Stykes:

Rundar, Rorar’s son strake the beast,

until the axe of his name was no more.

Nothing could pierce it in Horhild Hall,

only Mithril kept it at bay.

But Krazadol, mighty Krazadol,

he took up the Armorer’s Maul,

and not to shape shields.

With that great Yew he stove its head,

its spatter singed his sturdy arms,

and all rushed forth within the Cluun

to pelt the Beast with wrathful stones.

It wrothe and spat but bowed to sleep, 

and smiling Rundar, Rorar’s son,

then stooped and took its beak to keep.

Today, Dwarven miners assessing old mines hunt down local Rust Monsters to examine their metal content.  The purity and quantity of ore in the mine can thus be determined.  Such inspections are matters of great public interest in Dwarven society.  Successful examinations are followed by a festival known as the “Augur Lode.”  The celebration culminates with mead-drinking contests and torch-lit, drunken assaults on any Rust Monsters unlucky enough to have stayed in the mine.

Dead Rust Monsters with high concentrations of valuable metals are worth significant coin.  And if the carcass is fresh, it is useful in the creation of arms and armor.

Dwarves prize Rust Monster beaks as axe heads, and intact examples of the proper size and shape can fetch high prices from knowledgeable smiths.  Rust Monster antennae retain their metal-detecting properties even when removed from their original owners, and can be used to dowse for ore and buried metals.

Special Units/Subtypes

Man Eater.  An uncommon type that has developed a particular taste for the iron in blood.  Unlike their brethren, Man Eaters are found close to settlements and industry.  They prefer fresh blood to metal, and their slime glands produce a substance uniquely adapted to dissolve animal tissue.  Man Eaters will target any living creature that stumbles into their territory, but have a particular fondness for Dwarves.  They can sense Dwarven blood from great distances and will aggressively hunt down Dwarves that enter their domain.

Mage Eater.  Another uncommon type, Mage Eaters are extraordinarily sensitive to magical energy, so much that the presence of magic items and spell books overwhelms their senses and drives them into frenzy.  They will track and attack any being that has recently used magic or touched a magic item, ignoring even pure metal in their path.  Once on the scent, Mage Eaters are dogged and difficult to disengage, especially when magically active classes are present.

Flickering Stalker.  A very rare Rust Monster with an incandescent, silver-infused hide that catches and scatters incoming light.  Though beautiful to behold, this skin is of immense durability and not easily penetrated.  Flickering Stalkers have been driven mad by huge doses of silver, and are slaves to their hunger for it.  No longer interested in base metals, their senses are attuned to detect even tiny flecks of silver from long distances.  Once sensed, Flickering Stalkers will relentlessly pursue the silver source.  They may stalk for days, hiding and waiting for the right moment.  When they strike, they do so with speed and ferocity beyond that of their lesser brethren.  Flickering Stalker hides are prized by armors and fetch high prices.  The silver fibers within them have been mixed with natural secretions, forming an alloy that is flexible, relatively light, and extremely tough.  In the Common tongue, Stalker hides are sometimes called “Duergar Mithril” or “Undermere Mithril.”

Hummer.  “Humming” Rust Monsters use painful high-frequency sound to drive their prey toward them.  Hummers emit a constant, ear-piercing blast of sound that rattles teeth and deafens the ears.  PCs who enter the area between 100 feet and 60 feet of a Hummer must Save DC 15 CON or suffer -2 to AC, hit rolls, and ability Checks.  This causes most adventures to move out of the sound-affected ring as rapidly as possible.  Paradoxically, the sound Hummers emit decreases the closer one gets to them, driving many adventurers, and their precious metals, right into the expectant antennae of a Rust Monster.

Natural Weaponry

Melee 1.  Claws.  Each leg is tipped with a retractable metal claw that Rust Monsters use like a spear. 1d8+4 damage.

Melee 2.  Beaks.  Rust Monsters have a diamond-hard beak powerful enough to crush stone into dust, and to cleanly shear a broadsword blade.  2d10+6 damage.

Ranged 1.  Antennae/Tentacles.  These powerful tendrils can bludgeon a target up to 20′ away, and rend it with hooked barbs.  1d8+4 damage, may use to Grapple.

Ranged 2.  Corrosive Slime.  Rust Monster slime is corrosive to both metal and flesh, and deals 2d8+4 acid damage.  They spit their slime up to 30′.

Special Abilities

Slime Armor (All Variants).  Rust Monsters destabilize all non-Mithral, non-Adamantine metal that touches their slime layer.  If the attack is ranged, the ammunition is lost and cannot be recovered.  If the attack is melee, there is a 25% chance per hit that the slime will permanently reduce the weapon’s effectiveness by -1 to hit and -1 damage, stacking with each hit until the weapon is useless.  Magical weapons receive a DC 13 Save to resist this effect, adding their magical bonus to the Save roll.

Metal Seeker (All Variants).  A Rust Monster’s tentacles are strong and agile, able to grip and pull metal objects they find interesting.  Once per round as a full attack action, the monster may target a PC possessing metal items within 10 feet.  On a successful hit, the PC must win an opposed STR check vs an 18 STR, or have one metal ripped from their grasp.  The Rust Monster has a 50% chance to consume that item by the end of the round, unless it is made of rare metals or contains glass.  If the item is not consumed, it is flung away d20 feet in a random direction.  If consumed, the item has a 50% chance of being recovered upon the Rust Monster’s death.

Magic Seeker (Mage Eater, Flickering Stalker).  Magic-sensitive variants seek magic items.  Once per round as a full attack action, the monster may target a PC possessing magic items within 10 feet.  On a successful hit, the PC must win an opposed STR check vs an 18 STR, or have one magic item ripped from their grasp.  The Rust Monster has a 50% chance to consume that item by the end of the round, unless it is made of rare metals or contains glass.  If the item is not consumed, it is flung away d20 in a random direction.  If consumed, the item has a 75% chance of being recovered upon the Rust Monster’s death.

Blood Rust (Man Eater, Flickering Stalker).  A specialized gland on some variants produces a sticky, stinging slime.  Once per round, the Rust Monster may use its tentacles to fling gobs of this slime at up to four targets within a 20-foot radius.  If hit, targets take 1d8+4 damage, and 1d8+4 damage each round until they spend an action to remove the slime.  Metal items in contact with the slime must pass a DC 13 check, adding any magical bonuses, or become damaged.

Movement

Run.  Good.  Rust Monsters can move as fast as humans, over much worse terrain.

Climb.  Good.  Rust Monsters are adept at scaling jagged cave walls.

Swim.  Terrible.  Rust Monsters cannot swim, and will struggle not to drown in water deeper than their neck.

Burrow.  Excellent.  Rust Monsters can move rapidly through dirt or sand, and can burrow through solid rock.

Fly.  n/a

Grudges

In combat, Rust Monsters prioritize Dwarves, metal, and magic items.

See the full blog here.

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